Finding a tube sub that'll fit is probably the best way to go.

I suppose you could build a custom sealed enclosure to match the specs of the specific speaker you use, then power it with a seperate amp, but my guess is you aren't going to compete in sound pressure contests.. you probably just want a little bottom end to your music to help fatten out the sound.

Just remember these old trucks tend to rattle.. the more bass you add, the more you'll be chasing down rattling pieces of the truck.

an idea is only stupid if you think about it rationally.